1. Field of the Invention
The present invention relates to a process for rolling tubes in a continuous multi-stand rolling mill operating with a mandrel.
2. State of the Art
Longitudinal multi-stand rolling mills operating with a mandrel of the known art may be conventionally grouped into various types, according to their architecture and paying special attention to the control of the rolling speed and to the speed and position of the mandrel within the tube.
Continuous rolling mills with floating mandrel, i.e. free mandrel, are those in which the mandrel may freely move within the tube when passing in the multi-stand rolling mill for the rolling operation, according to the friction forces which are generated between the mandrel and the inner wall of the tube. Hence, the mandrel accelerates as the rolling stands sequentially take hold. The extraction of the mandrel from the tube occurs at the end of the rolling operation, outside the rolling line, or in any event when the tail of the tube has left the last rolling stand, and therefore when the free mandrel has taken the same feeding speed as the tube. Very short cycle times and hence high productivity, e.g. 4-5 pieces per minute, are obtained with these types of rolling mills.
On the other hand, this type of rolling mill is subject to various drawbacks. The mandrel acceleration causes states of compression in the tube which are detrimental to the dimensional quality and the defectiveness of the tubes, because the groove delimited by the rolling rollers is plugged (which status is conventionally referred to as “overfilling”) in the first stands and is choked (which status is referred to as “underfilling”) in the finishing stands at the end of the rolling mill. Therefore, problems of rolling stability and of products with too large tolerances are encountered. Moreover, the tube cooling over the length of the tube is uneven because the head part of the tube no longer reached by the mandrel remains hot longer right after the first rolling step, while the rear part where the mandrel is still inserted as the rolling operation continues is partly cooled by the mandrel with which it is in contact. In these rolling mills, there is normally the need to provide a heating furnace downstream in order to standardize the temperature of the tube before the final rolling operation which serves to calibrate or further decrease the tube diameter.
A second type of rolling mill is that called “semi-retained-mandrel rolling mill”, in which the mandrel is retained and fed more slowly than the tube, at the technologically favourable speed during the rolling operation. At the end of the rolling operation, once the tube tail has left the last rolling stand, the mandrel is released from the retaining device while remaining within the tube and following it while it is moved away from the rolling line. The extraction of the mandrel from the tube occurs outside the rolling line, or in any event when the tube tail has left the last rolling stand, and therefore when the free mandrel has taken the same feeding speed as the tube. Very short cycle times and hence high productivity, e.g. 3-4 tubes per minute, are obtained with this type of rolling mills.
On the other hand, equivalent problems are encountered with rolling mills of the above type with regards to the non-evenness of the temperature along the tube. A third type of rolling mill is that called “retained-mandrel rolling mill”, which is characterized by a device for retaining the rack-and-pinion mandrel. At the end of a tube rolling operation, when the tube tail leaves the last rolling mill stand, the tube has already been previously grasped at the head portion downstream of the rolling mill by an extracting device, which grasps the outer surface of the tube. The extracting device, which is generally in the shape of a particular sequence of roller rolling stands, drags the tube forwards in the same direction as the rolling, while the retaining system blocks the mandrel so that it is extracted from within the tube, and it pulls it backwards towards the inlet side of the rolling mill from where it is then unloaded and put back into the classical mandrel transport cycle. The extracting device or rolling mill also serves the function of decreasing the outer diameter of the tube by further rolling it without the inner mandrel when this has been extracted. The cycle times are longer in this type of rolling mill and therefore it has less productivity than the previously described types: 2 tubes per minute may generally be rolled.
In traditional rolling operations in retained-mandrel plants, during the rolling step the mandrel is fed at controlled speed, also called retaining speed, directed in the same motion direction as the tube, from the inlet to the outlet of the multi-stand rolling mill during the complete rolling cycle.
Normally, at the beginning of each rolling cycle in rolling processes implemented with this type of rolling mill, the mandrel is first inserted into the hollow body at the tail, in the direction of the head of the same hollow body with motion in the same direction as the direction of the tube rolling.
This first operation may occur in-line with the rolling axis, in this case it is called in-line insertion, or out-of-line, in this case it is called pre-insertion, as pre-inserting the mandrel into the hollow body is used to decrease the travel of the mandrel retaining devices, thus decreasing the cycle time of the rolling mill and increasing its productivity. Therefore, a limit in this technology is its low productivity, in particular for the rolling mills used for rolling small and medium tubes, e.g. those with nominal diameter less than or equal to 7″ (177.8 mm).
Another type of rolling mill is that called “retained-mandrel rolling mill” with extractor and with tube release at the end of the rolling operation, with the mandrel passing through the extractor. The rolling process carried out in this type of rolling mill provides that at the end of the tube rolling operation, the mandrel is immobilized by the specific retaining device while the tube is extracted from the mandrel by means of the extracting device by pulling it along the rolling line. Once the hollow body has completely passed through the extracting device, the mandrel is then released from the retaining device, conveyed forwards by pressing rollers along the rolling line, and is caused to pass through the extracting device immediately after the tube and lastly unloaded downstream of the extractor to follow the circuit arranged for reusing the mandrels. Relatively short cycle times (2.5 tubes per minute) are obtained in this type of rolling mills.
A drawback of this type of rolling mill is that the process includes conveying the mandrel, which is still very hot, by means of pressing rollers with the risk of damaging the mandrel surface. In this type of process, the mandrel retaining device in the rolling step, normally of rack type, is to provide a releasing device which operates in cycle, adapted to release the mandrel after the extraction of the tube.
To carry out the rolling process in a retained-mandrel rolling mill, passing the mandrel through the extracting device or rolling mill requires that the latter is provided with a stand which can rapidly open and close to first allow the rolled tube and then the mandrel to pass at each rolling cycle, given the high speeds at which tubes and mandrels move along the rolling line. If accuracy is not ensured in this operation of the extracting device, the risk may occur of misaligning the corners of two adjacent rollers and then longitudinally marking the rolled tube.
Processes with retained-mandrel rolling mills are advantageous with regards to the quality of the tube obtained and the thermal conditions with which the tube leaves the rolling mill, indeed only in this type of rolling mill the calibration to the final diameter of the tube may be provided even without intermediate heating.
To also ensure an efficient rolling process, both of retained and semi-retained type, it is worth arranging a retaining device which ensures the stability of the mandrel speed, is sturdy, and provides the possibility of hooking and releasing the mandrel without a chain and star system. Indeed, in the case of a rolling mill with semi-retained or retained mandrel, the device having a chain wound on stars and equipped with hooking gears is not advantageous due to the premature wear which occurs, to the noisiness and lengthening of the chain itself as operating time passes. To obviate such drawbacks of the chain system, in some known plants of retained-mandrel and controlled-speed type, in-cycle hooking and releasing systems were provided to develop rolling methods with a low cycle time. However, these systems do not operate centered with the pull axis of the mandrel, and therefore problems arise associated with bending loads acting on the hooking/releasing systems.
A rolling mill and retained-mandrel rolling process associated thereto is disclosed in document WO2011/000819, where after the extraction of the tube while the mandrel is still retained and the tube is transported and rolled through the extracting device without the mandrel therein, the mandrel is laterally removed from the rolling line with respect to the rolling line. However, in the known retained-mandrel plants described above, it is difficult to produce short tubes as the latter are shorter than the distance between the axis of the last stand of the multi-stand rolling mill and the first stand of the extractor.
The market requires rolling plants which allow increased flexibility of final product, i.e. are capable of rolling tubes of various lengths, with replacement operations over a minimum amount of components of the plant, which allow the tube rolling cycle time to be decreased and the overall productivity of the plant to be enhanced, which increase the quality of the finished tube or at least do not penalize it, which have a more rational structure than the plant itself, thus decreasing the production and handling cost thereof.
Generally, longitudinal rolling mills of the above-described types are also defined according to parameters such as:                Number of rollers per rolling stand (generally 2 or 3),        Possibility, or not, of loading the hollow body on the inlet side of the rolling mill with the mandrel already pre-inserted or with the mandrel inserted in-line,        Presence, or not, of calibrating stands of the tube upstream of the first rolling stand,        Presence, or not, of rounding stands arranged downstream of the last rolling stand, which roll the thickness of the tube between rollers and mandrel still arranged within the tube. Normally, the rounding stands are used in those rolling processes in which the extraction of the mandrel from the tube is performed out-of-line.        